System for stopping spindle at commanded position

ABSTRACT

During operation of a machine tool, a motor 14 driving a spindle 17 holding a tool 18 is operated at a commanded speed by a speed control system in accordance with a speed command signal VCMD with the feedback of an actual speed signal TSA. As a stopping command ORCM is issued for stopping the spindle, the spindle is decelerated to a predetermined speed in response to a reference signal output from a constant voltage generating circuit 33. After the deceleration of the spindle to the predetermined speed, as the rotational position of the spindle comes into a region within 180° from a predetermined position, a position offset signal PS&#39;, which smoothly crosses the zero level at the position at which the spindle is to be stopped, and which is generated from two signals e 1  and e 2  from a resolver connected to the spindle. Then, the operation mode is switched to a position control system in which the position offset signal is used as the reference signal for stopping the spindle at the commanded position.

BACKGROUND OF THE INVENTION

The present invention broadly relates to a spindle control system foruse in a machine tool which automatically performs mechanical machiningwhile making automatic exchange of tools and, more particularly, to asystem incorporating a resolver and adapted to stop the spindle of themachine tool at a commanded position. Still more particularly, theinvention is concerned with a system for stopping a spindle at acommanded position, improved to permit an adjustment of the stoppingposition.

Such machine tools are known as having automatic tool exchangingfunctions and are capable of automatically performing mechanicalmachining through automatic exchange of tools. In this type of machinetool, it is essential that the spindle be stopped precisely at acommanded position, in order that the mating portions of the spindle andthe tool can smoothly engage with each other. The stopping of thespindle at the commanded position is necessary also in boring, becausethe boring tool cannot be correctly inserted into a bore formed in theworkpiece unless the spindle is stationed precisely at the commandedposition. Thus, in the field of mechanical work, there is a great demandfor in the art for stopping the spindle of a machine tool precisely at acommanded position. Hitherto, a mechanical braking mechanism and a pinmechanism in combination have been used for stopping the spindle at thecommanded position. This mechanical system, however, often failed tostop the spindle at the commanded position partly because of frequentbreakage of the pin mechanism (stopping mechanism) due to theapplication of an abnormal external force or the occurrence of anextraordinary state in the machine tool and partly because of rapid wearof the braking mechanism due to friction. As a consequence, conventionalmachine tools often failed to perform the automatic exchange of toolsand the insertion of the boring tool. In order to avoid such a failure,it has been necessary to undertake a frequent inspection of the machinetool or a renewal of worn parts.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a spindlestopping system capable of stopping a spindle at a commanded positionwith a high precision purely electrically, i.e. by means of a resolver.

The invention also has an object of providing a system for stopping aspindle at a commanded position, improved to permit a fine adjustment ofthe stopping position within a range of about ±5°.

According to the invention, the spindle of a machine tool carrying anddriving various tools is driven at a commanded speed under the controlof a speed control system. Then, as a spindle stopping command is givenfor stopping the spindle at a commanded position, the control isswitched from the speed control system to a position control system todecelerate the spindle from a normal machining speed to a predeterminedlow speed. As the spindle is decelerated to the predetermined low speed,an output obtained through synchronous rectification of one of theoutput voltages from the primary coil of a resolver and an outputobtained by converting the other output voltage of the primary coil intoa rectangular wave are combined with each other to form a positiomoffset signal which smoothly crosses the zero level when the spindlerotates across the commanded stop position. This position offset signalis used as the reference signal in the position control system to makethe spindle gradually approach the commanded position. Then, as thespindle comes into a predetermined rotational region for stopping, anin-position signal is produced and, after confirming that the spindle iscorrectly stopped at the commanded position, an orientation completionsignal is produced to start the next step for tool exchange. Accordingto the invention, therefore, it is possible to maintain the correctpositional relationship between the tool on the spindle and theworkpiece to permit a smooth and safe exchange of tools.

According to another aspect of the invention, means is provided forfinely adjusting the lattice point at which the zero level is crossed bythe composite output of the resolver within a range of about ±5°, inorder to permit compensation for error which may be involved in themounting of the resolver on the spindle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment of the invention;

FIGS. 2 and 3 are waveform charts showing waveforms of signals availableat various parts in the embodiment shown in FIG. 1;

FIG. 4 is a circuit diagram of a position adjusting circuit; and

FIG. 5 is an illustration of position adjustment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention will be fully described hereafter with reference to theaccompanying drawings. Referring first to FIG. 1, a speed commandcircuit 11 incorporated in a numerical controller NC produces a speedcommand V_(CMD). A speed control circuit 12 includes a phasecompensation circuit, voltage/phase converter and a thyristor circuit(not shown). An adder/subtracter circuit 13 is adapted to produce, inthe speed control mode of operation, an output voltage signal (speedoffset) corresponding to the difference between the commanded speedV_(CMD) and actual speed TSA and, in the stop position control mode ofoperation, a difference voltage signal corresponding to alater-mentioned position offset and the actual speed TSA. Atachogenerator 15 is adapted to produce an actual speed voltage signalTSA corresponding to the speed of a D.C. motor 14. Reference numerals16, 17 and 18 denote a gear box, spindle and a tool, respecively. Aclutch 19 housed by the gear box 16 is operated when a speed changinggear 20 between the D.C. motor 14 and the spindle 17 is switched betweena "Low" mode and a "High" mode.

A resolver 21 has primary coils arranged to cross each other at a rightangle and a secondary coil wound on a rotor. The secondary coil receivesan exciting signal which is generated by a later-mentioned resolverexciting signal generator. The secondary coil generates, in accordancewith the offset of the rotor, offset signals e₁ and e₂ having a phasedifferential π/2.

Representing the exciting signal and the offset angle by A.sin.wt and θ,respectively, these offset signals e₁ and e₂ can be expressed asfollows.

    e.sub.1 =A.sin θ.sin wt                              (1)

    e.sub.2 =A.cos θ.cos wt                              (2)

Reference numerals 22 and 23 denote a clock oscillator and a timingcircuit, respectively. The aforementioned resolver exciting signalgenerator is designated as reference numeral 24. Numerals 25, 26, 27 and28 are used to denote, respectively, a synchronous rectifier, a quadrantdiscrimination circuit, a waveform composition circuit and a stopposition adjusting circuit.

The clock oscillator 22 continuously produces a clock signal CL in theform of rectangular pulses at an oscillation frequency of 3 kHz. Thisclock signal is fed to the resolver exciting signal generator 24 throughthe timing circuit 23. The resolver exciting signal generator 24produces a resolver exciting signal of 3 kHz and delivers the same tothe secondary coil of the resolver 21. As the secondary coil receivesthe resolver exciting signal of 3 kHz in the form of a sine wave, theprimary coils produce the signals e₁ and e₂ determined by the formulae(1) and (2) above. More specifically, the signal e₁ is a sine-wavesignal of 3 kHz having an amplitude which changes in proportion to thesine of the angle θ of rotation of the rotor of the resolver 21, whilethe signal e₂ is a cosine-curve signal of 3 kHz having an amplitudewhich changes in proportion to the cosine of the rotation angle θ, aswill be seen from FIG. 2. The sine-wave signal e₁ is delivered to thesynchronous rectifier circuit 25 which makes a synchronous rectificationof the sine-wave signal by means of the 3 kHz clock signal deliveredthrough the timing circuit 23, thereby to produce a rotational positionsignal RPS (thick solid line curve in e₁ in FIG. 2) which changes in theform of a sine curve in accordance with the rotation of the spindle 17.The signal RPS is delivered to a waveform combining circuit 27.Meanwhile, the cosine-wave signal e₂ is fed to the quadrantdiscrimination circuit 26 which then effects synchronous rectificationof the signal e₂ to produce a signal which changes in the form of acosine-wave in accordance with the rotation of the spindle 17 as shownby thick solid line in e₂ in FIG. 2. This cosine-wave signal is thenconverted into two quadrant discrimination signals Q₁ and Q₂ havingrectangular waveforms and having a 180° phase differential from eachother. Upon receipt of the rotational position signal RPS and thequadrant discrimination signals Q₁ and Q₂ ' the waveform compositioncircuit 27 produces therein a rectangular signal RTS of an amplitude V₁and combines this rectangular signal RTS and the rotational positionsignal RPS (amplitude V₁) to produce a position signal PS or PS'. Thepoint at which the zero level is crossed by the position signal PS' isgenerally referred to as a "lattice point" GP. The position signal PS'smoothly crosses the lattice point each time the spindle 17 makes onefull rotation, from the positive side to the negative side or vice versain accordance with the direction of rotation of the spindle 17. Thespindle is stopped at this lattice position, i.e. at the position wherethe position signal PS' is zero. The lattice point is adjustable bymeans of the stop position adjusting circuit 28. Namely, the waveformcomposition circuit AMP 27 incorporates an amplifier circuit (See FIG.4) for amplifying the position signal PS'. The amplifier circuit has anamplifier AMP the offset voltage of which is adjustable by apotentiometer POT incorporated in the position adjusting circuit 28.Namely, the position signal PS' is shifted to the left and right toshift the stopping position as the offset voltage V_(o) is controlled byadjustment of the potentiometer POT as will be seen from FIG. 5.

A speed detection circuit 29 is adapted to produce a speed signal VRSwhen the rotation speed of the DC motor has reached a predeterminedspeed, i.e. when the output from the tachogenerator has reached apredetermined level. A zero-speed detector 30 produces a speed zerosignal when the output of the tachogenerator becomes zero. A rotationdirection discriminator 31 is adapted to check whether the outputvoltage from the tachogenerator 15 is positive or negative and toproduce a rotation direction signal RDS which takes a "1" level and a"0" level in response to forward rotation and backward rotation,respectively. A reference numeral 32 denotes a direction changingcircuit (DDC) while 33 designates a constant voltage generating circuit(const.). The constant voltage generating circuit 33 produces a constantvoltage of +Vi (volts) when the forwardly rotating spindle is to bestopped and a constant voltage -Vi (volt) when the spindle to be stoppedis rotating backwardly. A switch change-over circuit (SC) 34 is adaptedto turn switches SW4 and SW5 on and off, respectively, when it receivesa spindle stopping command ORCM from a spindle stopping command means(not shown). When the speed of the D.C. motor reaches the predeterminedlevel, i.e. when the speed signal VRS takes the level "1", the switchchange-over circuit turns the switches SW4 and SW5 off and on,respectively. A gain adjusting circuit (GA) 35 is adapted to increasethe gain of the position control loop when the low gear is used, i.e.when the speed reduction ratio is high, while another gain adjustingcircuit 36 has a function to decrease the gain of the position controlloop when the high gear is used, i.e. when the speed reduction ratio islow. A switch change-over circuit (SC) 37 is adapted to turn a switchSW2 on while turning off switches SW1 and SW3, when the spindle stoppingcommand ORCM is logical "1", i.e., "ON" (closed) while the low gear isin use, that is, while the signal CLH is at the level "1". However, whenthe spindle stopping command ORCM takes the "1" level while the highgear is in use, i.e., while the signal CLH is at the "0" level, theswitch change-over circuit 37 turns the switch SW3 on while turning theswitches SW1 and SW2 off. When the spindle stopping command ORCM takesthe "0" level, i.e. when it is in the OFF state (open), the switch SW1is turned on while the switches SW2 and SW3 are turned off. A referencenumeral 38 designates an in-position signal generator composed of acomparator. The in-position signal generator 38 monitors the positionsignal PS' and produces an in-position signal INPOS when the spindle hascome into a predetermined rotational region around the commandedposition. An orientation completion signal generating circuit 39produces an orientation completion signal ORAR when the spindle rotationspeed becomes zero (VZR="1") followed by the changing of the in-positionsignal INPOS to the "1" level, on condition that the spindle stoppingcommand ORCM takes the "1" level.

The system of the invention having the described construction operatesin a manner explained hereafter.

When the spindle is rotating for the cutting of a workpiece or otherpurpose, the switch SW1 is held in an on state while the switches SW2and SW3 are in off state to complete the speed control loop. Namely, thesubtracter circuit 13 receives the speed command VCMD from the speedcommand circuit 11 and the actual speed TSA from the tachogenerator 15,and produces a speed offset voltage signal. The speed control circuit 12controls the firing angle of a thyristor (not shown), in accordance withthe speed offset voltage signal, thereby to increase or decrease thevoltage impressed on the D.C. motor 14. In consequence, the actual speedTSA of the D.C. motor 14 is increased or decreased to the level of thespeed command VCMD. Thus, the speed control is effected to nullify thespeed offset, so that the spindle is driven at the commanded speed.

Then, as the cutting of the work by the machine tool is finished, thespindle stopping command ORCM is issued by the spindle stopping commandmeans (not shown) at a moment t₀ (See FIG. 3). Upon receipt of thespindle stopping command, the switch change-over circuit 37 turns on oneof the switches SW2 and SW3 while turning off the switch SW1, dependingon the state of the gear 20, i.e. depending on whether the signal CLHtakes the level "1" or "0", thereby to complete a position control loop.Once this loop is formed, the constant voltage generated by the constantvoltage generating circuit 33 is delivered to the adder/subtractercircuit 13 through the switch SW4, either one of the gain adjustingcircuits 35 and 36 and either one of the switches SW2 and SW3. As aconsequence, the actual spindle speed is decreased following the brokenline curve in FIG. 3 and is lowered to the predetermined speed at amoment t₁. Meanwhile, the rotational direction detection circuit 31 ismonitoring whether the actual speed voltage TSA is positive or negative,and produces a rotational direction signal RDS which takes the level "1"(forward rotation) when the actual speed voltage is positive and thelevel "0" (backward rotation) when the same in negative. For instance,assuming here that the spindle is rotating forwardly during cutting, therotational direction signal RDS takes the level "1" so that the constantvoltage generating circuit 33 produces a constant positive voltage(+Vi). The level of the constant positive voltage Vi is equal to themaximum amplitude of the position signal PS'.

When the spindle is decelerated to the predetermined speed, the speeddetection circuit 29 produces the speed signal VRS of "1" level. As adetection means (not shown) detects that the spindle has a rotationalposition which is within 180° of the commanded position after the levelof the speed signal VRS is changed to "1", the switch change-overcircuit 34 turns the switch SW4 off while turning the switch SW5 on. Theoutput voltage +Vi from the constant voltage generating circuit 33applied to the adder/subtracter circuit 13 is replaced by the positionsignal PS' (See FIG. 2) coming from the waveform composition circuit 27.Namely, the position signal PS' is delivered to the adder/subtractercircuit 13 through the switch SW5, either one of the gain adjustingcircuits 35 and 36 and either one of the switches SW2 and SW3. Theadder/subtracter circuit 13 then effects a position control to nullifythe difference between the position signal PS' and the actual speed TSA.As the rotational position of the spindle becomes sufficiently close tothe commanded position, the in-position signal generating circuit 38produces an in-position signal INPOS. Thereafter, as the spindle speedbecomes zero, the orientation completion signal generating circuit 39produces an orientation completion signal ORAR.

As has been described, according to the invention, a resolver 21 is usedas the position detector and the output of the resolver is utilizedeffectively in such a manner that the position offset signal crosses thezero level smoothly (at an acute angle) to permit the stopping of thespindle precisely at the commanded position.

It is, however, difficult to correctly attach the resolver 21 to thespindle 17 in such a manner that the position signal PS' crosses thezero level at the designated position. This gives rise to a demand forfine adjustment to make the position signal PS' cross the zero voltlevel at the correct position, after attaching the resolver to thespindle. This can be accomplished by shifting the lattice point GP ofthe position signal PS' to the left and right as shown in FIG. 5(a) byadjusting the potentiometer POT (See FIG. 4) of the stopping positionadjusting circuit 28. By so doing, it is possible to adjust thecommanded spindle stopping position within a region of rotational angleof about +5° and, hence, to make the position signal PS' cross the zerolevel at the commanded position.

It is preferred that the length of time required for orientation besubstantially constant regardless of the state of the speed reductiongears, i.e. independently of whether the low gear is used or the highgear, and that the system be stabilized. To this end, according to theinvention, the gain of the position control loop is so controlled that ahigh gain is obtained when the low gear is used while a low gain isobtained when the high gear is used, by the selective use of the gainadjusting circuits 35 and 36 and the switches SW2 and SW3 in combinationwith the switch change-over circuit 37. By changing the gain inaccordance with the state of the gear (speed reduction ratio), it ispossible to eliminate any hunting or overshoot in the stopping of thespindle and to make the time length required for the stopping of thespindle substantially constant regardless of the speed reduction ratio.

To shorten the length of time for stopping the spindle at the commandedposition, it is preferred that control for stopping the spindle at thecommanded position be performed not only when the spindle is rotatingforwardly but also when the spindle is rotating backwardly. To cope withthis demand, the system of the invention incorporates the rotationaldirection detection circuit 31 and the constant voltage generatingcircuit 33 which produces a constant positive voltage +Vi and a constantnegative voltage -Vi when the spindle is rotating forwardly andbackwardly, respectively, and control is effected such that the level ofthe position signal PS' approaches from the positive level +Vi to thezero level when the spindle is rotating forwardly and from the negativelevel -Vi to the zero level when the spindle is rotating backwardly.

As has been described, a novel spindle stopping system incorporating aresolver is provided by the present invention. Since the commandedspindle stopping position can be simply adjusted within a region ofabout ±5° even after mounting the resolver on the spindle, it ispossible to stop the spindle at the commanded position with a highdegree of accuracy, even if there is an unavoidable error in themounting of the resolver. In addition, since the gain of the positioncontrol loop can be changed over in accordance with the state of thespeed reduction gear, it is possible to stabilize the system and to makethe length required for stopping substantially constant, regardless ofthe state of the speed reduction gear. Furthermore, since control forstopping the spindle at the commanded position can be made not only whenthe spindle is rotating forwardly but also when the spindle is rotatingbackwardly, it is possible to shorten the length of time required forstopping the spindle at the commanded position. This system, therefore,can be used suitably in a machine tool having an automatic toolexchanging function for performing mechanical machining whileautomatically exchanging tools.

What is claimed is:
 1. A system for stopping a spindle at a commandedposition operating at a commanded speed, comprising:an electric motor; aspeed detector, operatively connected to said electric motor, fordetecting a speed of rotation of said electric motor and for producingan actual speed signal; a speed control circuit, operatively connectedto said speed detector and to receive the commanded speed, for effectingcontrol to nullify the offset of the actual speed from the commandedspeed; a spindle operatively connected to and driven by said electricmotor; and a position control circuit, operatively connected to saidspindle and said speed control circuit, for generating a position offsetsignal corresponding to an offset of a position of a predeterminedportion of said spindle from the commanded position at which saidportion of said spindle is to be stopped, the rotation of said spindlebeing controlled by said speed control circuit to maintain saidcommanded speed and by said position commanded to nullify said positionoffset signal, said position control circuit comprising:a resolver,operatively connected to said spindle, having a secondary coil andhaving two primary coils producing output voltages; a resolver excitingsignal generator, operatively connected to said resolver, for supplyingan exciting signal to the secondary coil of said resolver; a synchronousrectifier circuit, operatively connected to one of the primary coils ofsaid resolver, for providing synchronous rectification of the outputvoltage from said one of said primary coils; a quadrant discriminationcircuit, operatively connected to the other of said primary coils, forconverting the output voltage from said other of said primary coils intorectangular waves; a waveform combining circuit, operatively connectedto said synchronous rectifier circuit and said quandrant discriminationcircuit, for combining the output voltage of said synchronous rectifiercircuit and the output voltage from said quadrant discrimination circuitto produce said position offset signal which smoothly crosses a zerolevel at the commanded position at which said spindle is to be stopped;and a stopping position adjusting circuit, operatively connected to saidwaveform combining circuit, for adjusting a position of a lattice pointat which said position offset signal crosses the zero level.
 2. A systemfor stopping a spindle at a commanded position according to claim 1,wherein a gain of the position control system is switched in accordancewith a low or high state of a gear positioned between said motor andsaid spindle.
 3. A system for stopping a spindle at a commanded positionaccording to claim 2, further including a circuit, operatively connectedto said speed control circuit, for stopping said spindle from a presentdirection of rotation of said spindle.
 4. A system for stopping aspindle, operatively connectable to receive a commanded position signal,comprising:positioning means, operatively connected to the spindle andoperatively connectable to receive the commanded position signal, forpositioning the spindle; and adjustable offset means, operativelyconnected to said positioning means, for generating an adjustable offsetposition signal allowing the stopping position of the spindle to beadjusted in both the forward and backward rotational directions, saidpositioning means positioning the spindle at a commanded position independence upon the adjustable offset position signal and the commandedposition signal.
 5. A system for stopping a spindle according to claim4, wherein said adjustable offset means comprises:excitation means forgenerating an excitation signal; a resolver, operatively connected tosaid excitation means and the spindle, for generating spindle positionsignals in dependence upon the spindle position by modifying theexcitation signal; a rectifier circuit, operatively connected to saidexcitation means and said resolver, for generating a rotational positionsignal in dependence upon one of the spindle position signals; aquadrant discrimination circuit, operatively connected to said resolver,for generating quadrant signals in dependence upon another one of saidspindle position signals; a stop position adjustment circuit forgenerating an adjustable offset signal; and a combining circuit,operatively connected to said positioning means, said rectifier circuit,said quadrant discrimination circuit and said stop position adjustmentcircuit, for combining the rotational position signal, the quadrantsignals and the offset signal to produce the adjustable offset positionsignal.
 6. A system for stopping a spindle according to claim 5, whereinsaid excitation means comprises:an oscillator; a timing circuitoperatively connected to said oscillator and said rectifier circuit; andan excitation signal generator operatively connected to said timingcircuit and said resolver.